On an inner peripheral surface of a large end such as a con rod of an automobile, for example, a technology of forming a fracture start portion composed of recessed portions in a groove state or linearly continuing at slight intervals at positions opposing each other in a thickness direction has been known (see Patent Literature 1 and Patent Literature 2, for example).
When such a fracture start portion is to be formed, in Patent Literature 1 and Patent Literature 2, a laser device is used. In order to form an accurate fracture start portion by the laser device, a considerably expensive device at a price of several tens of millions yen needs to be introduced and installed. Thus, instead of this laser device, an exclusive broaching machine is used for forming a fracture start portion having a V-shaped groove in some cases (see Patent Literature 3, for example).